NWCG Introduction to Wildland Fire Behavior Calculations (S-390) Practice Test

The effect of slope on rate of spread comes from how heat and flames reach fuels ahead of the fire. When the fire moves uphill, radiant heat and convective currents preheat and dry fuels above the fire more effectively, so the next patch of fuel ignites sooner and the front advances faster. Gravity and flame geometry also help keep the heat transfer aligned with the upslope fuels, boosting spread. Downhill, preheating is reduced and the flame front encounters less readily ignitable fuel, slowing the spread. In models this is expressed with a slope factor: it increases the baseline rate of spread on uphill terrain and decreases it on downhill terrain (level ground is about 1). So upward slope increases ROS, downward slope can reduce ROS, and the slope factor is used to account for this in predictions.